This invention relates to a mechanical vibrator for use as an optical chopper in a photosensor.
Conventional tuning fork type vibrators driven by a piezoelectric method have two functions such as an electrical filter and a mechanical vibrator. To operate the conventional vibrators at a stable frequency and to prevent it from spurious vibrations, the vibrators should be softly mounted. In case of employing the tuning fork type vibrator as an optical chopper, it is necessary to mount the vibrator such that a center position of a vibrating plate of the vibrator is kept at an exact center of a light beam to be chopped, in order to obtain a stable on-off ratio. To this end, the vibrating plate of the vibrator should be tightly held and, otherwise, it is difficult to operate the vibrator at an stable frequency. This invention intends to solve these two subjects opposite to each other, by constructing the tuning fork structure with two thin vibrating plates. When the conventional tuning fork vibrator is employed as an optical chopper in a photosensor, retaining the vibrator in a stable manner is difficult because of its shape. This will become clear from the following description of a conventional tuning fork vibrator, with reference being made to the accompanying drawings.
FIG. 1 shows an example of a conventional tuning fork vibrator which is employed as an optical chopper, and FIGS. 2 and 3 show examples of methods through which the tuning fork vibrator of FIG. 1 may be retained.
Referring now to FIG. 1, vibrating plates 11 and 12 are shown brazed to a spacer 13. Electrodes 17 and 18 and piezoelectric members 14 and 15 for receiving electric signals are bonded to respective ones of the vibrating plates 11 and 12. The piezoelectric members 14 and 15 are connected to an amplifier (not shown), one to the input side and one to the output side thereof, by means of the electrodes 17 and 18, and an electrode 16 is connected to ground. This is the arrangement for inducing vibration in the plates 11 and 12. A light-chopping member 19 is either manufactured separately and then attached to the plate 11, or is formed unitarily with the plate 11. The same is true for a balancing member 20 which is provided on the plate 12 to counterbalance the light-chopping member 19.
The tuning fork vibrator shown in FIG. 1 is retained by inserting the spacer portion in a hole which is formed in a rubber member 21, as illustrated in FIG. 2. This may be accomplished alternatively by employing metal plates 22 and 23 to embrace the spacer portion of the vibrator, with screws 24 and 25 being used to fasten the plates tightly together as shown in FIG. 3. Before proceeding further it should be noted that the tuning fork vibrator of FIGS. 2 and 3 is shown pointing in the direction opposite to that of FIG. 1.
The method of retention shown in FIG. 2 makes it difficult to retain the tuning fork vibrator stably at a fixed position since the rubber member 21 is employed as the retaining means. With the arrangement of FIG. 3, on the other hand, vibration of one of the plates, say the plate 11, is attenuated by the metal plates 22 and 23 and is not transmitted to the plate 12. This reduces the amplitude of vibration of the vibrator and occasionally makes it impossible to initiate of an oscillation system including the vibrator and an amplifier.
A conventional tuning fork vibrator devised to prevent the attenuation of vibrations is exemplified by the arrangement shown in FIG. 4. Here the plates 11 and 12 are formed by bending a single strip of metal, and a supporting rod 26 is spot-welded to the center of the bent portion, the supporting rod 26 being soldered to a pedestal 27. With this arrangement, however, problems are encountered in retaining the plates 11 and 12 stably since the supporting rod 16 must be slender enough to prevent attenuation of vibration.